lose of erection at the moment of penetration

Hi, im jman and im a healthy 21 year old male. i use to smoke, but i stopped, because i heard smoking affect your sex drive. I have 3 months without smoking now, and i feel great. I have a 6.5 inch penis, and a great body, very athletic and i feel like i can get any girl, but i dont. I dont have sex, but im not a virgin. i had sex in four occasions with two different experienced girls when i was 18. Those girls were just friends, no strings attached. At the moment of intimacy with them, touching, kissing, and feeling each others body parts, ill have a rock hard erection. When it was time to have sex, ill get up and position myself to penetrate, and at the nomen im about to insert, i will go limp. Now as i mentioned those girls are experienced, and i wasn't, so they helped me out, and with the time we had, i managed to have sex four times in total with the two girls. Yea, intercourse was quick. Ill cum like in 10 minuets. After my experience with those two girls, i haven't had sex since. Every now and then ill get a blowjob, but no sex. I masturbate ALOT, still in present day i masterbate ALOT. now, i want to be a good lover in bed, satisfy women, but i figured the only way to be good at sex is if you practice, and im lacking practice. im 21 years old now and i still havent had sex. I meet this girl last year whom i became fond of. She is beautiful, smart, and has a sexy figure, and best part, she is a virgin. Since we meet, we found each other attractive, so we started dating. as beautiful as this girl is. She is so unexperience to the extreme. i tried to talk about sex with her, but she feels like she is not ready. Well, i dont want to rush her, because i love the girl, and since I've never loved before, and neither has she, i want to sex and our love to be right. We been together officially for 8 months now. Everything is great. We it comes to sexual stuff, im teaching her a few things. I go down on her alot and she loves it. Theres times when i try to finger her and insert my finger, but when i do she says it hurts her, so i just rub her p***y, and she gets really wet not to mention. She is suppppper tight. our sexual comfort together is good. theres no shyness between us. We dry hump alot and she goes down on me too, and i get rock hard. i put my penis on her p***y, an d we dry hump alot, we just havent had sex. Sometimes i feel that its going to happen, and as i position my self to insert. My penis cant go in, because how tight she is. my intentions are to put my head in so she can like it, and we finally have sex. But when im about to insert and it doesnt go in, i start getting limp? It happens all the time. i dont why? I wondered maybe is because i never been with a virgin and as nervouse as she is im as nervous too, or my lack of sexual potency to hold an erection as soon as she stops touching me. I dont what it is, but i want to be real good at sex and i want to have ny practice with her. Can someone tell me whats going on? Why do i lose the rock hard erection as soon as we slow things down? Please someone help me. Its frustrating and i still havent had sex :(.. Thank you